Apparatus of plural charged-particle beams

ABSTRACT

A multi-beam apparatus for observing a sample with high resolution and high throughput is proposed. In the apparatus, a source-conversion unit changes a single electron source into a virtual multi-source array, a primary projection imaging system projects the array to form plural probe spots on the sample, and a condenser lens adjusts the currents of the plural probe spots. In the source-conversion unit, the image-forming means is on the upstream of the beamlet-limit means, and thereby generating less scattered electrons. The image-forming means not only forms the virtual multi-source array, but also compensates the off-axis aberrations of the plurality of probe spots.

CLAIM OF PRIORITY

This application is a continuation of application Ser. No. 16/167,429,filed Oct. 22, 2018; which is a divisional of application Ser. No.15/633,639, filed Jun. 26, 2017, now issued as U.S. Pat. No. 10,109,456;which is a continuation of application Ser. No. 15/403,749, filed Jan.11, 2017, now issued as U.S. Pat. No. 9,691,586; which is a divisionalof application Ser. No. 15/065,342, filed Mar. 9, 2016, now issued asU.S. Pat. No. 9,691,588; which claims the benefit of priority of U.S.provisional application No. 62/130,819 entitled to Ren et al. filed Mar.10, 2015, and entitled “Apparatus of Plural Charged-Particle Beams”, Thedisclosures of the above-referenced applications are incorporated hereinby reference in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a charged-particle apparatus with aplurality of charged-particle beams. More particularly, it relates to anapparatus which employs plural charged-particle beams to simultaneouslyacquire images of plural scanned regions of an observed area on a samplesurface. Hence, the apparatus can be used to inspect and/or reviewdefects on wafers/masks with high resolution and high throughput insemiconductor manufacturing industry.

2. Description of the Prior Art

For manufacturing semiconductor IC chips, pattern defects and/oruninvited particles (residuals) inevitably appear on surfaces ofwafers/masks during fabrication processes, which reduce the yield to agreat degree. To meet the more and more advanced requirements onperformance of IC chips, the patterns with smaller and smaller criticalfeature dimensions have been adopted. Accordingly, the conventionalyield management tools with optical beam gradually become incompetentdue to diffraction effect, and yield management tools with electron beamare more and more employed. Compared to a photon beam, an electron beamhas a shorter wavelength and thereby possibly offering superior spatialresolution. Currently, the yield management tools with electron beamemploy the principle of scanning electron microscope (SEM) with a singleelectron beam, which therefore can provide higher resolution but can notprovide throughputs competent for mass production. Although the higherand higher beam currents can be used to increase the throughputs, thesuperior spatial resolutions will be fundamentally deteriorated byCoulomb Effect.

For mitigating the limitation on throughput, instead of using a singleelectron beam with a large current, a promising solution is to use aplurality of electron beams each with a small current. The plurality ofelectron beams forms a plurality of probe spots on one being-inspectedor observed surface of a sample. For the sample surface, the pluralityof probe spots can respectively and simultaneously scan a plurality ofsmall scanned regions within a large observed area on the samplesurface. The electrons of each probe spot generate secondary electronsfrom the sample surface where they land on. The secondary electronscomprise slow secondary electrons (energies ≤50 eV) and backscatteredelectrons (energies close to landing energies of the electrons). Thesecondary electrons from the plurality of small scanned regions can berespectively and simultaneously collected by a plurality of electrondetectors. Consequently, the image of the large observed area includingall of the small scanned regions can be obtained much faster thanscanning the large observed area with a single beam.

The plurality of electron beams can be either from a plurality ofelectron sources respectively, or from a single electron source. For theformer, the plurality of electron beams is usually focused onto andscans the plurality of small scanned regions by a plurality of columnsrespectively, and the secondary electrons from each scanned region aredetected by one electron detector inside the corresponding column. Theapparatus therefore is generally called as a multi-column apparatus. Theplural columns can be either independent or share a multi-axis magneticor electromagnetic-compound objective lens (such as U.S. Pat. No.8,294,095). On the sample surface, the beam interval between twoadjacent beams is usually as large as 30˜50 mm.

For the latter, a source-conversion unit is used to virtually change thesingle electron source into a plurality of sub-sources. Thesource-conversion unit comprises one beamlet-forming means and oneimage-forming means. The beamlet-forming means basically comprises aplurality of beam-limit openings, which divides the primary electronbeam generated by the single electron source into a plurality ofsub-beams or beamlets respectively. The image-forming means basicallycomprises a plurality of electron optics elements, which either focusesor deflects the plurality of beamlets to form a plurality of parallelimages of the electron source respectively. Each of the plurality ofparallel image can be taken as one sub-source which emits onecorresponding beamlet. The beamlet intervals, i.e. the beam-limitopening intervals are at micro meter level so as to make more beamletsavailable, and hence the source-conversion unit can be made bysemiconductor manufacturing process or MEMS (Micro Electro MechanicalSystems) process. Naturally, one primary projection imaging system andone deflection scanning unit within one single column are used toproject the plurality of parallel images onto and scan the plurality ofsmall scanned regions respectively, and the plurality of secondaryelectron beams therefrom is respectively detected by a plurality ofdetection elements of one electron detection device inside the singlecolumn. The plurality of detection elements can be a plurality ofelectron detectors placed side by side or a plurality of pixels of oneelectron detector. The apparatus therefore is generally called as amulti-beam apparatus.

In the source-conversion unit 20-1 in FIG. 1A, the image-forming means22-1 is composed of a plurality of lenses (22_1L˜22_3L). Thesubstantially parallel primary electron beam 2 from one single electronsource is divided into the plurality of beamlets (2_1˜2_3) by theplurality of beam-limit openings (21_1˜21_3) of the beamlet-formingmeans 21, and the plurality of lenses respectively focuses the pluralityof beamlets to form the plurality of parallel images (2_1 r˜2_3 r) ofthe single electron source. The plural parallel images are typicallyreal images, but can be virtual images in specific conditions if each ofthe plurality of lenses is an aperture lens. U.S. Pat. Nos. 7,244,949and 7,880,143 respectively propose an multi-beam apparatus with oneimage-forming means of this type. In the source-conversion unit 20-2 inFIG. 1B, the image-forming means 22-2 is composed of a plurality ofdeflectors (22_2D and 22_3D). The divergent primary electron beam 2 fromone single electron source is divided into the plurality of beamlets(2_2 and 2_3) by the plurality of beam-limit openings (21_2 and 21_3) ofthe beamlet-forming means 21, and the plurality of deflectorsrespectively deflects the plurality of beamlets to form a plurality ofparallel virtual images (2_2 v and 2_3 v) of the single electron source.

The concept of using a deflector to form a virtual image of an electronsource was used in the famous two-slit electron interference experimentsas early as in 1950s, wherein an electron biprism is employed to formstwo virtual images as shown in FIG. 2 (FIG. 1 of the paper “TheMerli-Missiroli-Pozzi Two-Slit Electron-Interference Experiment”published in Physics in Perspective, 14 (2012) 178-195 by Rodolfo Rosa).The electron biprism basically comprises two parallel plates at groundpotential and a very thin wire F therebetween. When a potential notequal to ground potential is applied to the wire F, the electron biprismbecomes two deflectors with deflection directions opposite to eachother. The primary electron beam from the electron source S passes thetwo deflectors and becomes two deflected beamlets which form the virtualimages S1 and S2 of the electron source S. If the potential is positive,the two beamlets overlap with each other and the interference fringesappear in the overlapping area.

Since then, the foregoing concept has been employed in a multi-beamapparatus in many ways. JP-A-10-339711 and U.S. Pat. No. 8,378,299directly use one conventional electron biprism to form two probe spotson the sample surface. U.S. Pat. No. 6,943,349 uses one annulardeflector (its FIG. 5) or one corresponding deflector array (its FIG.12) to form more than two probe spots on the sample surface andtherefore can provide a higher throughput. The annular deflectorincludes an inner annular electrode and an outer annular electrode. Ifthe potentials of the two annular electrodes are not equal to eachother, one electric field in the local radial direction will appearwithin the annular gap therebetween, and consequently the annulardeflector can deflect more than two beamlets together in differentdirections. Furthermore, the deflection function of the annulardeflector can be performed by one corresponding deflector array whichhas a plurality of multi-pole type deflectors arranged along the annulargap.

In the conventional source-conversion unit 20-2 in FIG. 1B, due to thedivergence of the primary electron beam 2, the plurality of beamletspasses through the plurality of beam-limit openings with differentangles of incidence and therefore suffers strong and different electronscatterings. The scattering electrons in each beamlet will enlarge theprobe spot and/or become a background noise and therefore deterioratethe image resolution of the corresponding scanned region.

In U.S. Pat. No. 6,943,349, the current of the plurality of beamlets canonly be changed by varying either the emission of the single electronsource or the sizes of the beam-limit openings. The single electronsource takes a long time to become stable when the emission thereof isvaried. The beamlet-forming means needs to have more than one group ofopenings and the sizes of the openings of one group are different fromthe other groups. It is very time-consuming to change the group in use.In addition, the secondary electron beams can only be focused onto themultiple detection elements of the in-lens detector in some specificoperation conditions of the objective lens. Therefore the availableapplications are limit.

Accordingly, it is necessary to provide a multi-beam apparatus which cansimultaneously obtain images of a plurality of small scanned regionswithin a large observed area on the sample surface with high imageresolution and high throughput. Especially, a multi-beam apparatus whichcan inspect and/or review defects on wafers/masks with high resolutionand high throughput is needed to match the roadmap of the semiconductormanufacturing industry.

SUMMARY OF THE INVENTION

The object of this invention is to provide a new multi-beam apparatuswhich can provide both high resolution and high throughput for observinga sample and especially functioning as a yield management tool toinspect and/or review defects on wafers/masks in semiconductormanufacturing industry. The multi-beam apparatus employs a newsource-conversion unit to form a plurality of parallel virtual images ofa single electron source at first and limit the currents of thecorresponding plurality of beamlets secondly, a condenser lens to adjustthe currents of the plurality of beamlets, a primary projection imagingsystem to project the plurality of parallel virtual images to form aplurality of probe spots on a being-observed surface of the sample, abeam separator to deflect a plurality of secondary electron beamstherefrom away from paths of the plurality of beamlets, and a secondaryprojection imaging system to focus the plurality of secondary electronbeams to be detected respectively by a plurality of detection elementsof an electron detection device.

Accordingly, the invention therefore provides a source-conversion unit,which comprises an image-forming means which comprises an upper layerwith a plurality of upper 4-pole structures and a lower layer with aplurality of lower 4-pole structures, and a beamlet-limit means which isbelow the image-forming means and comprises a plurality of beam-limitopenings. Each upper 4-pole structure is above and aligned with onecorresponding lower 4-pole structure, and both have about 45° differencein azimuth and form a pair of 4-pole structures. Therefore the pluralityof upper 4-pole structures and the plurality of lower 4-pole structuresform a plurality of pairs of 4-pole structures. The plurality ofbeam-limit openings is aligned with the plurality of pairs of 4-polestructures respectively. One pair of 4-pole structure functions as amicro-deflect to deflect one beamlet of an electron beam generated bythe electron source to form a virtual image thereof, a micro-lens tofocus one beamlet to a desired degree, and/or a micro-stigmator to add adesired amount of astigmatism aberration to one beamlet.

The present invention also provides a multi-beam apparatus for observinga surface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, a primary projection imaging system below thesource-conversion unit and comprising an objective lens, a deflectionscanning unit inside the primary projection imaging system, a samplestage below the primary projection imaging system, a beam separatorabove the objective lens, a secondary projection imaging system abovethe beam separator, and an electron detection device with a plurality ofdetection elements. The source-conversion unit comprises animage-forming means with a plurality of micro-deflectors and abeamlet-limit means with a plurality of beam-limit openings, in whichthe image-forming means is above the beamlet-limit means. The electronsource, the condenser lens, the source-conversion unit, the primaryprojection imaging system, the deflection scanning unit and the beamseparator are aligned with a primary optical axis of that apparatus. Thesample stage sustains the sample so that the surface faces to theobjective lens. The secondary projection imaging system and the electrondetection device are aligned with a secondary optical axis of theapparatus, and the secondary optical axis is not parallel to the primaryoptical axis. The electron source generates a primary electron beamalong the primary optical axis, and the plurality of micro-deflectorsdeflects the primary electron beam to form a plurality of parallelvirtual images of the electron source. Therefore a virtual multi-sourcearray is converted from the electron source, and a plurality of beamletswhich includes the virtual multi-source array passes through theplurality of beam-limit openings respectively. A current of each beamletis therefore limited by one corresponding beam-limit opening, andcurrents of the plurality of beamlets can be varied by adjusting thecondenser lens. The primary projection imaging system images the virtualmulti-source array onto the surface and a plurality of probe spots istherefore formed thereon. The deflection scanning unit deflects theplurality of beamlets to scan the plurality of probe spots respectivelyover a plurality of scanned regions within an observed area on thesurface. A plurality of secondary electron beams is generated by theplurality of probe spots respectively from the plurality of scannedregions and in passing focused by the objective lens. The beam separatorthen deflects the plurality of secondary electron beams to the secondaryprojection imaging system, and the secondary projection imaging systemfocuses and keeps the plurality of secondary electron beams to bedetected by the plurality of detection elements respectively. Eachdetection element therefore provides an image signal of onecorresponding scanned region.

The multi-beam apparatus may further comprise a main aperture platebelow the electron source, which has a main opening aligned with theprimary optical axis and functions as a beam-limit aperture for theprimary electron beam. The primary projection imaging system may furthercomprise a transfer lens above the objective lens, which focuses theplurality of beamlets to land on the surface perpendicularly. Each ofthe plurality of micro-deflectors has a 4-pole structure which cangenerate a deflection field in any radial direction. The multi-beamapparatus may further comprise a single-beam electron detector above thebeam separator, which can be used in a single-beam mode. The multi-beamapparatus may further comprise an in-lens electron detector with abeamlet-passing hole aligned with the primary optical axis, which isbelow the beam separator and can be used in the single-beam mode.

The present invention also provides a multi-beam apparatus for observinga surface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, a primary projection imaging system below thesource-conversion unit and comprising an objective lens, a deflectionscanning unit inside the primary projection imaging system, a samplestage below the primary projection imaging system, a beam separatorabove the objective lens, a secondary projection imaging system abovethe beam separator, and an electron detection device with a plurality ofdetection elements. The source-conversion unit comprises animage-forming means with a plurality of micro-deflector-and-compensatorelements and a beamlet-limit means with a plurality of beam-limitopenings, and each micro-deflector-and-compensator element comprises onemicro-deflector and one micro-compensator which have one micro-lens andone micro-stigmator. The image-forming means is above the beamlet-limitmeans. The electron source, the condenser lens, the source-conversionunit, the primary projection imaging system, the deflection scanningunit and the beam separator are aligned with a primary optical axis ofthe apparatus. The sample stage sustains the sample so that the surfacefaces to the objective lens. The secondary projection imaging system andthe electron detection device are aligned with a secondary optical axisof the apparatus, and the secondary optical axis is not parallel to theprimary optical axis. The electron source generates a primary electronbeam along the primary optical axis, and the plurality ofmicro-deflectors deflects the primary electron beam to form a pluralityof parallel virtual images of the electron source. Therefore a virtualmulti-source array is converted from the electron source. A plurality ofbeamlets which includes the virtual multi-source array passes throughthe plurality of beam-limit openings respectively, and a current of eachbeamlet is therefore limited by one corresponding beam-limit opening.Currents of the plurality of beamlets can be varied by adjusting thecondenser lens. The primary projection imaging system images the virtualmulti-source array onto the surface and a plurality of probe spots istherefore formed thereon. The one micro-lens and the one micro-stigmatorof the one micro-compensator respectively compensates field curvatureand astigmatism aberrations of one corresponding probe spot, and thedeflection scanning unit deflects the plurality of beamlets to scan theplurality of probe spots respectively over a plurality of scannedregions within an observed area on the surface. A plurality of secondaryelectron beams is generated by the plurality of probe spots respectivelyfrom the plurality of scanned regions and in passing focused by theobjective lens, and the beam separator then deflects the plurality ofsecondary electron beams to enter the secondary projection imagingsystem. The secondary projection imaging system focuses and keeps theplurality of secondary electron beams to be detected by the plurality ofdetection elements respectively, and each detection element thereforeprovides an image signal of one corresponding scanned region.

The multi-beam apparatus may further comprise a main aperture platebelow the electron source, which has a main opening aligned with theprimary optical axis and functions as a beam-limit aperture for theprimary electron beam. Each of the plurality ofmicro-deflector-and-compensator element may have an 8-pole structurewhich functions as the one micro-deflector by generating a desireddeflection field, and the one micro-compensator by generating a desiredquadrupole field and a desired round-lens field. Each of the pluralityof micro-deflector-and-compensator element comprises an upper 4-polestructure and a lower 4-pole structure in an upper layer and a lowerlayer respectively, the upper layer is above the lower layer, and theupper 4-pole structure and the lower 4-pole structure are aligned witheach other and have a 45° difference in azimuth. The upper 4-polestructure and the lower 4-pole structure may function as the onemicro-deflector by generating a desired deflection field and the onemicro-compensator by generating a desired quadrupole field and a desiredround-lens field. The primary projection imaging system may furthercomprise a transfer lens above the objective lens, which focuses theplurality of beamlets to land on the surface perpendicularly. Themulti-beam apparatus may further comprise a single-beam electrondetector above the beam separator, which can be used in a single-beammode. The multi-beam apparatus may further comprise an in-lens electrondetector with a beamlet-passing hole aligned with the primary opticalaxis, which is below the beam separator and can be used in thesingle-beam mode.

The present invention also provides a method to configure asource-conversion unit for forming a virtual multi-source array from anelectron source, which comprises steps of providing an image-formingmeans which comprises an upper layer with a plurality of upper 4-polestructures and a lower layer with a plurality of lower 4-polestructures, and providing a beamlet-limit means which is below theimage-forming means and comprises a plurality of beam-limit openings.Each upper 4-pole structure is above and aligned with one correspondinglower 4-pole structure, and both have a 45° difference in azimuth andform a pair of 4-pole structures. Therefore the plurality of upper4-pole structures and the plurality of lower 4-pole structures form aplurality of pairs of 4-pole structures. The plurality of beam-limitopenings is aligned with the plurality of pairs of 4-pole structuresrespectively. One pair of 4-pole structure functions as a micro-deflectto deflect one beamlet of an electron beam generated by the electronsource to form a virtual image thereof, a micro-lens to focus the onebeamlet to a desired degree, and/or a micro-stigmator to add a desiredamount of astigmatism aberration to the one beamlet.

The source-conversion unit may comprise an upper electric-conductionplate with a plurality of upper through-holes respectively aligned withthe plurality of pairs of 4-pole structures. The source-conversion unitmay further comprise a lower electric-conduction plate with a pluralityof lower through-holes respectively aligned with the plurality of pairsof 4-pole structures.

The present invention also provides a method for forming a virtualmulti-source array from an electron source, which comprises steps ofdeflecting an electron beam from the electron source into a plurality ofbeamlets by using an upper layer with a plurality of upper 4-polestructures and a lower layer with a plurality of lower 4-polestructures, and limiting the plurality of beamlets by using a pluralityof openings. Each upper 4-pole structure is above and aligned with onecorresponding lower 4-pole structure, and both have a 45° difference inazimuth and form a pair of 4-pole structures.

The present invention also provides a charged-particle beam apparatus,which comprises a single charged particle source for providing a primarybeam, means for converting the primary beam into a plurality ofbeamlets, a first projection system for forming a plurality of probespots on a specimen from the plurality of beamlets, a deflectionscanning unit for scanning the plurality of probe spots on the specimen,means for separating a plurality of signal electron beams away from theplurality of beamlets, a detection device for receiving the plurality ofsignal electron beams, and a second projection system for forming aplurality of signal-spots from the plurality of signal electron beamsrespectively on a plurality of electron detection elements of thedetection device. The converting means comprising a plurality ofdeflectors for deflecting the plurality of beamlets and a plurality ofbeam-limiting openings under the plurality of deflectors. The pluralityof signal electron beams is respectively generated due to the pluralityof beamlets bombarding on the specimen

The charged-particle beam apparatus may further comprise a condenserlens for adjusting currents of the plurality of probe spots. Theconverting means comprises a plurality of compensators for compensatingaberrations of the plurality of probe spots respectively.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIGS. 1A and 1B are respectively a schematic illustration of aconventional source-conversion unit.

FIG. 2 is a schematic illustration of the electron interferenceexperiment with an electron biprism.

FIG. 3A is a schematic illustration of one configuration of a newmulti-beam apparatus in accordance with one embodiment of the presentinvention.

FIGS. 3B˜3D are respectively schematic illustrations of operation modesof the new multi-beam apparatus in FIG. 3A.

FIG. 4 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 5A˜5C are respectively a schematic illustration of a configurationof an image-forming means in FIG. 3A in accordance with anotherembodiment of the present invention.

FIGS. 6A˜6D are respectively a schematic illustration of a configurationof an image-forming means in FIG. 3A in accordance with anotherembodiment of the present invention.

FIG. 7 is a schematic illustration of a configuration of an advancedimage-forming means in FIG. 4 in accordance with another embodiment ofthe present invention.

FIGS. 8A˜8D are schematic illustrations of a configuration of anadvanced image-forming means in FIG. 4 in accordance with anotherembodiment of the present invention.

FIG. 9A is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 9B and 9C are respectively schematic illustrations of operationmodes of the new multi-beam apparatus in FIG. 9A.

FIG. 10 is a schematic illustration of one operation mode of the newmulti-beam apparatus in FIG. 4.

FIG. 11A is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention.

FIG. 11B is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described. For sake ofclarity, only three beamlets are available in the drawings, but thenumber of beamlets can be anyone.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of alens (round or multi-pole), an imaging system or an apparatus”, “radial”means “in a direction perpendicular to the optical axis”, “on-axial”means “on or aligned with the optical axis” and “off-axis” means “not onor not aligned with the optical axis”.

In this invention, “an imaging system is aligned with an optical axis”means “all the electron optics elements (such round lens and multipolelens) are aligned with the optical axis”.

In this invention, X, Y and Z axe form Cartesian coordinate. The opticalaxis of the primary projection imaging system is on the Z-axis, and theprimary electron beam travels along the Z-axis.

In this invention, “primary electrons” means “electrons emitted from anelectron source and incident onto a being-observed or inspected surfaceof a sample, and “secondary electrons” means “electrons generated fromthe surface by the “primary electrons”.

In this invention, “signal electrons” means “electrons generated from abeing-observed or inspected surface of a sample by a primary chargedparticle beam”.

In this invention, “single-beam mode” means only one beamlet is in use.

In this invention, all terms relate to through-holes, openings andorifices mean openings or holes penetrated through one plate.

Next, the present invention will provide some embodiments of a newmulti-beam apparatus. The multi-beam apparatus employs a newsource-conversion unit to form a plurality of parallel virtual images ofa single electron source at first and limit the currents of a pluralityof beamlets secondly, a condenser lens to adjust the currents of theplurality of beamlets, a primary projection imaging system to projectthe plurality of parallel virtual images to form a plurality of probespots on a being-observed surface of the sample, a beam separator todeflect a plurality of secondary electron beams therefrom away frompaths of the plurality of beamlets, and a secondary projection imagingsystem to focus the plurality of secondary electron beams to be detectedrespectively by a plurality of detection elements of an electrondetection device.

The new source-conversion unit comprises an image-forming means with aplurality of micro-deflectors and a beamlet-limit means with a pluralityof beam-limit openings, and the image-forming means is on the upstreamof the beamlet-limit means. The primary electron beam from a singleelectron source is at first deflected by the plurality ofmicro-deflectors to form a plurality of parallel virtual images of thesingle electron source, and the plurality of beamlets forming theplurality of parallel virtual images will pass through the plurality ofbeam-limit perpendicularly or substantially perpendicularly. In thisway, the plurality of beam-limit openings will not only generate fewerscattered electrons than the prior of art, but also cut off thescattered electrons generated on the upstream, and thereby eliminatingthe image resolution deterioration due to the electron scattering. Theimage-forming means can further comprise a plurality ofmicro-compensators to compensate off-axis aberrations (field curvatureand astigmatism) of the plurality of probe spots respectively andthereby further improving the image resolution of the being-observedsurface.

One embodiment 100A of the new multi-beam apparatus is shown in FIG. 3A.The single electron source 101 is on the primary optical axis 100_1. Thecommon condenser lens 110, the main aperture plate 171, the newsource-conversion unit 120, the primary projection imaging system 130,the deflection scanning unit 132 and the beam separator 160 are placedalong and aligned with the primary optical axis 100_1. The secondaryprojection imaging system 150 and the electron detection device 140 areplaced along and aligned with the secondary optical axis 150_1.

The main aperture plate 171 can be placed above the common condenserlens 9, or immediately above the new source-conversion unit 120 as shownhere. The new source-conversion unit 120 comprises the micro-deflectorarray 122 with two micro-deflectors 122_2 and 122_3, and a beamlet-limitplate 121 with three beam-limit openings 121_1, 121_2 and 121_3, whereinthe beam-limit opening 121_1 is aligned with the primary optical axis100_1. If the beam-limit opening 121_1 is not aligned with the primaryoptical axis 100_1, there will be one more micro-deflector 122_1 (asshown in FIG. 5C). The primary projection imaging system 130 comprises atransfer lens 133 and an objective lens 131. The deflection scanningunit 132 comprises at least one deflector. The beam separator 160 is oneWien Filter. The secondary projection imaging system 15 o comprises ananti-scanning deflector 151, a zoom lens 152 (comprising at least twolenses 152_1 and 152_2) and an anti-rotation magnetic lens 154. Theelectron detection device 140 comprises three detection elements 140_1,140_2 and 140_3. Each of the foregoing lenses can be an electrostaticlens, a magnetic lens or an electromagnetic compound lens.

FIGS. 3B-3D shows three operation modes of the new multi-beam apparatus100A. The single electron source 100 comprises a cathode, an extractionand/or an anode, wherein the primary electrons are emitted from thecathode and extracted and/or accelerated to form a primary electron beam102 with high energy (such as 8˜20 keV), a high angular intensity (suchas 0.5˜5 mA/sr) and a crossover (virtual or real) 101 s shown by theon-axis oval mark here. Therefore it is convenient to think that theprimary electron beam 102 is emitted from the crossover 101 s, and thesingle electron source 101 is simplified to be the crossover 101 s.

In FIG. 3B, the condenser lens 110 is off. The primary electron beam 102passes through the condenser lens 110 without focusing influence and itsperipheral electrons are cut off by the main opening of the mainaperture plate 171. The micro-deflectors 122_2 and 122_3 respectivelydeflect beamlets 102_2 and 102_3 of the primary electron beam 102. Thedeflected beamlets 102_2 and 102_3 respectively form the off-axisvirtual images 102_2 v and 102_3 v of the crossover 101 s of the singleelectron source 101. The deflected beamlets 102_2 and 102_3 are parallelor substantially parallel to the primary optical axis 100_1 andtherefore perpendicularly incident onto the beamlet-limit plate 121. Thebeam-limit openings 121_1, 121_2 and 121_3 respectively cut off theperipheral electrons of the center part 102_1 of the primary electronbeam 102 and the deflected beamlets 102_2 and 102_3, and therebylimiting the currents thereof. Consequently, one virtual multi-sourcearray 101 v is formed, which comprises the crossover 101 s and its twoparallel off-axis virtual images 102_2 v and 102_3 v. One virtual imagecan avoid the Coulomb Effect at one real image in FIG. 1A. To furtherreduce Coulomb Effect, the main aperture plate 171 can be placed abovethe condenser lens 110 to cut off the peripheral electrons as early aspossible.

Next the crossover 101 s and its two parallel off-axis virtual images102_2 v and 102_3 v are imaged onto the being-observed surface 7 by thetransfer lens 133 and the objective lens 131, and their images formthree probe spots 102_1 s, 102_2 s and 102_3 s thereon. To make the twooff-axis beamlets 102_2 and 102_3 perpendicularly landing on thebeing-observed surface 7, the transfer lens 133 focuses them to passthrough the front focal point of the objective lens 131. If theobjective lens 131 comprises one magnetic lens, the two off-axisbeamlets 102_2 and 102_3 may not exactly pass through the front focalpoint due to the influence of magnetic rotation, and this is veryhelpful to reduce the Coulomb Effect at the beamlet crossover CS. Thedeflection scanning unit 132 deflects the three beamlets 102_1˜102_3 andconsequently the three probe spots 102_1 s˜102_3 s scan three individualregions on the being-observed surface 7.

Secondary electron beams 102_1 se, 102_2 se and 102_3 se emitted fromthe three scanned regions are focused by the objective lens 131 anddeflected by the beam separator 160 to enter the secondary projectionimaging system 150 along the secondary optical axis 150_1. The lenses152 and 153 focus the secondary electron beams onto the three detectionelements 140_1˜140_3 respectively. Therefore each detection element willprovide an image signal of one corresponding scanned region. If somesecondary electrons of the secondary electron beam from one scannedregion go to the neighboring detection elements, the image signals ofneighboring detection elements will also comprise the foreigninformation from this scanned region, and for the neighboring detectionelements the foreign information is a cross-talk from this scannedregion. To avoid the cross-talks among the detection elements, the zoomlens 152 make the spot size of each secondary electron beam smaller thanthe corresponding detection element, and the anti-scanning deflector 151will synchronously deflect the secondary electron beams 102_1 se˜102_3se to keep them within the corresponding detection elements during thedeflection scanning unit 132 deflects the beamlets 102_1˜102_3.

Different samples usually request different observing conditions, suchas the landing energies and the currents of the beamlets. This isespecially true for inspection and/or review of the defects onwafers/masks in semiconductor manufacturing industry. The focusing powerof the objective lens 131 will change with the landing energies, whichwill influence the positions of the secondary electron beams on theelectron detection device 140 and incur cross-talks. In this case, thezoom lens 152 will be adjusted to eliminate the radial displacements ofthe secondary electron beams. If the objective lens 131 comprises onemagnetic lens, the anti-rotation magnetic lens 154 will be adjusted toeliminate the rotation of the secondary electron beams.

Each of the two off-axis probe spots 102_2 s and 102_3 s comprises theoff-axis aberrations generated by the objective lens 131, the transferlens 133 and the condenser lens when being turned on. The off-axisaberrations of each off-axis probe spot can be reduced by individuallyoptimizing the trajectory of the corresponding beamlet. The static partsof the off-axis aberrations can be reduced by adjusting the deflectionpower of the corresponding micro-deflector. The dynamic parts of theoff-axis aberrations can be reduced by optimizing the performance of thedeflection scanning unit 132 which therefore may comprise more than onedeflector.

Different from FIG. 3B, the condenser lens 110 is turned on in FIG. 3C,which focuses the primary electron beam 102 to form an on-axis virtualimage 101 sv of the crossover lots of the single electron source lot.The micro-deflectors 122_2 and 122_3 respectively deflect beamlets 102_2and 102_3 of the focused primary electron beam 102, and form twooff-axis virtual images 102_2 v and 102_3 v of the crossover 101 s. Thedeflected beamlets 102_2 and 102_3 are parallel or substantiallyparallel to the primary optical axis 100_1 and therefore perpendicularlyincident onto the beamlet-limit plate 121. The beam-limit openings121_1, 121_2 and 121_3 respectively cut off the peripheral electrons ofthe center part 102_1 of the focused primary electron beam 102 and thedeflected beamlets 102_2 and 102_3, and thereby limiting the currentsthereof. The focusing function of the condenser lens 110 increase thecurrent density of the focused primary electron beam 102, and therebyincreasing the currents of the beamlets 102_1˜102_3 higher than in FIG.3B. Hence, the currents of all the beamlets can be continuously adjustedby the condenser lens 110.

Similar to a conventional SEM, the size of each probe spot is minimizedby balancing the geometric and diffraction aberrations, Gaussian imagesize and Coulomb effect. The focusing function of the condenser lens 110changes the imaging magnification from the crossover 101 s to thebeing-observed surface 7, which influences the balance and therefore mayincrease the size of each probe spot. To avoid largely increasing thesizes of the probe spots when the currents of the beamlets are largelyvaried, the sizes of the beam-limit openings 121_1˜121_3 can beaccordingly changed. Consequently, the beamlet-limit plate 121 ispreferred having multiple groups of beam-limit openings. The sizes ofbeam-limit openings in a group are different from those in anothergroup. Alternately, the focusing power of the transfer lens 133 can bechanged to reduce the variation of the imaging magnification. Thetrajectories of the off-axis beamlets 102_2 and 102_3 will be influencedby the focusing power variation of the transfer lens 133, and deflectionpowers of the micro-deflectors 122_2 and 122_3 can be accordinglyadjusted to keep the trajectories. In this way, the beamlets 102_2 and102_3 may be slightly not parallel to the primary optical axis 100_1, asshown in FIG. 3D.

Another embodiment 110A of the new multi-beam apparatus is shown in FIG.4. Different from the embodiment 100A, the new source-conversion unit120-1 comprises one micro-deflector-and-compensator array 122-1 withthree micro-deflector-and-compensator elements 122_1 dc, 122_2 dc and122_3 dc. Each micro-deflector-and-compensator element comprises onemicro-deflector and one micro-compensator having one micro-lens and onemicro-stigmator. The micro-deflectors are used to form one virtualmulti-source array, same as the functions of the micro-deflectors 122_2and 122_3 shown in FIGS. 3B˜3D. As well known, the condenser lens 110,the transfer lens 133 and the objective lens 131 will generate off-axisaberrations. As mentioned above, the influence of the off-axisaberrations on the sizes of the probe spots can be reduced byindividually optimizing the trajectories of the beamlets. Themicro-lenses and the micro-stigmators therefore will be used tocompensate the left field curvature and astigmatism aberrations of theprobe spots respectively. In comparison with the micro-deflector array122 in FIG. 3A, the micro-deflector-and-compensator array 122-1 is anadvanced image-forming means.

Each of the micro-deflectors 122_2 and 122_3 in FIG. 3A can simplyinclude two parallel electrodes perpendicular to the required deflectiondirection of the corresponding beamlet, as shown in FIG. 5-4. Forexample, the micro-deflector 122_2 has two parallel electrodes 122_2_e1and 122-2_e2 perpendicular to the X-axis and thereby deflecting thebeamlet 102_2 in the X-axis direction. FIG. 5B shows one embodiment ofthe micro-deflector array 122 to deflect eight beamlets. Due to eachmicro-deflector has a special orientation, it is difficult to make onemicro-deflector array 122 comprising a large number of micro-deflectors.From the manufacturing point of view, all the micro-deflectors arepreferred to have same configuration and same orientation in geometry.Hence a micro-deflector with a quadrupole or 4-pole configuration canmeet this requirement, as shown in FIG. 5C. Four electrodes of eachmicro-deflector can form two deflectors which can deflect one electronbeamlet in any direction. The micro-deflector 122_1 can be used if thecorresponding beam-limit opening 121_1 is not correctly aligned with theprimary optical axis 101.

To operate one micro-deflector, a driving-circuit needs connecting witheach electrode thereof. To prevent the driving-circuits from beingdamaged by the primary electron beam 102, it is better placing oneelectric-conduction plate above the electrodes of all themicro-deflectors in FIGS. 5A˜5C. Taking FIG. 5C as an example, in FIG.6A, an upper electric-conduction plate 122-CL1 with multiple upperthrough-holes and an upper insulator plate 122-IL1 with multiple upperorifices are placed above the electrodes of the micro-deflectors122_1˜122_3. The electrodes of the micro-deflectors 122_1˜122_3 can beattached to the upper insulator plate insulator plate 122-IL1. The upperthrough-holes and the upper orifices are aligned with the optical axesof the micro-deflectors respectively, such as the upper through-holeCL1_2 and the upper orifice IL1_2 are on the optical axis 122_2_1 of themicro-deflector 122_2. The radial size of each upper through-hole isequal to or smaller than the inner radial dimensions of the electrodesof the corresponding micro-deflector for protecting the driving-circuitsthereof, while the radial size of each upper orifice is larger than theradial size of the corresponding upper through-hole to avoid charging-upon the inner sidewall thereof. In this way, the deflection fields of allthe micro-deflectors will have short fringe ranges on the upper side,which will reduce the deflection aberrations thereof.

Based on FIG. 6A, the micro-deflector array 122 in FIG. 6B furthercomprises a lower electric-conduction plate 122-CL2 with multiple lowerthrough-holes. Each lower through-hole is aligned with the optical axisof one micro-deflector, such as the lower through-hole CL2_2 is on theoptical axis 122_2_1 of the micro-deflector 122_2. In this way, thedeflection fields of all the micro-deflectors will have short fringeranges on both upper and lower sides, which will reduce the deflectionaberrations thereof. Different from FIG. 6B, the micro-deflector array122 in FIG. 6C employs a lower insulator plate 122-IL2 with multiplelower orifices to support the electrodes of the micro-deflectors122_1˜122_3. Each lower orifice is aligned with the optical axis of onemicro-deflector, such as the lower orifice IL2_2 is on the optical axis122_2_1 of the micro-deflector 122_2. The radial size of each lowerorifice is larger than the inner radial dimensions of the electrodes ofthe corresponding micro-deflector. The micro-deflector array 122 in FIG.6D is a combination of FIG. 6B and FIG. 6C, which is more stable inconfiguration.

FIG. 7 shows one embodiment of the micro-deflector-and-compensatorelement 122_2 dc of the micro-deflector-and-compensator array 122-1 inFIG. 4, which has an 8-pole configuration. The eight electrodes 122_2dc_e1˜122_2 dc_e8 can be driven to generate a dipole field (deflectionfield) in any direction with a basic amount for generating a virtualimage of the electron source 1 and an additional amount for compensatingdistortion, a quadrupole field (astigmatism field) in any direction forcompensating astigmatism and a round-lens field for compensating fieldcurvature.

FIG. 8A shows another embodiment of the micro-deflector-and-compensatorarray 122-1 in FIG. 4. Each micro-deflector-and-compensator elementcomprises a pair of 4-pole lenses which are placed in two layers,aligned with each other and have a 45° difference in azimuth ororientation. The micro-deflector-and-compensator element 122_1 dc, 122_2dc and 122_3 dc are respectively composed of the pair of the upper andlower 4-pole lenses 122_1 dc-1 and 122_1 dc-2, the pair of the upper andlower 4-pole lenses 122_2 dc-1 and 122_2 dc-2, and the pair of the upperand lower 4-pole lenses 122_3 dc-1 and 122_3 dc-2. The upper 4-polelenses 122_1 dc-1, 122_2 dc-1 and 122_3 dc-1 are placed in the upperlayer 122-1-1, and the lower 4-pole lenses 122_1 dc-2, 122_2 dc-2 and122_3 dc-2 are placed in the lower layer 122-1-2 and respectivelyaligned with the upper 4-pole lenses 122_1 dc-i, 122_2 dc-1 and 122_3dc-1. As an example, with respect to the X axis, the azimuths of theupper 4-pole lenses 122_1 dc-i, 122_2 dc-1 and 122_3 dc-1 are 0° asshown in FIG. 8B, and the azimuths of the lower 4-pole lenses 122_1dc-2, 122_2 dc-2 and 122_3 dc-2 are 45° as shown in FIG. 8C. In FIG. 8D,similar to FIG. 6D, the upper and lower layers are shielded by the upperand lower electric-conduction plate 122-CL1 and 122-CL2, and supportedby the upper and lower insulator plates 122-IL1 and 122-IL2 and a middleinsulator plate 122-IL3 with multiple middle orifices. For eachmicro-deflector-and-compensator element, the deflection field in anydesired direction and the round-lens field can be generated by either orboth of the upper and lower 4-pole lenses, and the quadrupole field inany direction can be generated by both of the upper and lower 4-polelenses.

Another embodiment 200A of the new multi-beam apparatus is shown in FIG.9A. In comparison with the embodiment 110A in FIG. 4, the transfer lens133 is removed from the primary projection imaging system. FIG. 9B showsone operation mode, wherein the off-axis beamlets 102_2 and 102_3 arerespectively deflected parallel to the primary optical axis 200_1 by themicro-deflector-and-compensator elements 122_2 dc and 122_3 dc, andobliquely land on being-observed surface 7. This mode can be used to theobservation applications which have no strict requirements on theincident situations of the beamlets or require stereo-imaging. Themicro-deflector-and-compensator elements 122_2 dc and 122_3 de cancompensate the large off-axis aberrations of the two off-axis beamlets102_2 and 102_3 due to passing through the objective lens 131 with largeradial shifts. FIG. 9C shows another operation mode, wherein theoff-axis beamlets 102_2 and 102_3 are respectively further deflected bythe micro-deflector-and-compensator elements 102_2 dc and 102_3 dctowards the primary optical axis 200_1, and accordingly less obliquelyland on the being-observed surface 7. If themicro-deflector-and-compensator elements 122_2 dc and 122_3 dcrespectively deflect the off-axis beamlets 102_2 and 102_3 to passthrough the front focal point of the objective lens 131, the off-axisbeamlets 102_2 and 102_3 will be normally incident onto thebeing-observed surface 7. To avoid the off-axis beamlets 102_2 and 102_3passing through the beam-limit openings with large angles of incidence,it is preferred to keep a long distance between the front focal point ofthe objective lens 131 and the micro-deflector-and-compensator array122-1.

As well known, the more beamlets scan the being-observed surface 7, themore charges may be built thereon. Hence for a specific observationapplication, some of the beamlets may be not needed. In this case, thosebeamlets can be directed to be blanked by the beamlet-limit plate. FIG.10 shows such an operation mode of the embodiment 110A in FIG. 4,wherein the micro-deflector-and-compensator 122_2 dc is off and thebeamlet 102_2 is cut off by the beamlet-limit plate 121. Themicro-deflector-and-compensator 122_2 dc may need to be turned on todirect the beamlet 102_2 cut off by the beamlet-limit plate 121, whichis up to the detailed structure of the source-conversion unit 120-1.

Based on the embodiment 110A in FIG. 4, another embodiment 111A of thenew multi-beam apparatus is proposed in FIG. 11A, wherein a single-beamelectron detector 141 is added. When only one beamlet is needed for somereasons such as searching optimistic imaging conditions (landing energyand probe current) for an observation application, the apparatus willwork on a single-beam mode. In this case, beam-separator 160 can deflectthe corresponding secondary electron beam to the single-beam electrondetector 141. Here the beamlet 102_1 is taken as the beamlet in use. Thesecondary electron beam 102_1 se generated by the beamlet 102_1 isdeflected to be detected by the single-beam electron detector 141. Usingthe single-beam electron detector 141 can avoid the procedures ofadjusting the secondary projection imaging system 150 with respect tothe focusing power variation of the objective lens 131. As mentionedabove, the focusing power of the objective lens 131 will change when thelanding energy and/or current of the beamlet in use are changed.Furthermore, FIG. 11B shows one more embodiment 112A of the newmulti-beam apparatus, wherein an in-lens electron detector 142 with abeamlet-passing hole is placed below the beam separator 160. When theapparatus works in the single-beam mode, within the secondary electronbeam with respect to the beamlet in use, the secondary electrons withlarge emission angles can be detected by the in-lens electron detector142, and the secondary electrons with small emission angles will passthrough the beamlet-passing hole and be detected by the correspondingdetection element of the electron detection device 140. Here the beamlet102_1 is taken as the beamlet in use. Within the secondary electron beam10201 se generated by the beamlet 102_1, the secondary electrons 102_1se_2 with large emission angles hit the in-lens electron detector 142and the secondary electrons 102_1 se_1 with small emission angles aredeflected to be detected by the electron detection device 1400. Thesingle-beam electron detector 141 and the in-lens electron detector 142can be used in combination. In this case, the secondary electrons 102_1se_2 with large emission angles can be detected by the in-lens electrondetector 112 and the secondary electrons 102_1 se_1 with small emissionangles can be deflected by the beam separator 160 to be detected by thesingle-beam electron detector 141. Although it is not shown here, thein-lens electron detector 142 can also be placed above the beamseparator 160. In this case, the in-lens electron detector 142 candetect the outer part of the secondary electron beam 102_1 se when thebeam separator is off.

In summary this invention proposes a new multi-beam apparatus forobserving a sample with high resolution and high throughput. The newmulti-beam apparatus can function as a yield management tool to inspectand/or review defects on wafers/masks in semiconductor manufacturingindustry. The multi-beam apparatus employs a new source-conversion unitto form a plurality of parallel virtual images of a single electronsource, a condenser lens to adjust the currents of the plurality ofbeamlets, a primary projection imaging system to project the pluralityof parallel virtual images to form a plurality of probe spots on abeing-observed surface of the sample, a beam separator to deflect aplurality of secondary electron beams therefrom away from paths of theplurality of beamlets, and a secondary projection imaging system tofocus the plurality of secondary electron beams to be detectedrespectively by a plurality of detection elements of an electrondetection device. In the new source-conversion unit, the image-formingmeans is on the upstream of the beamlet-limit means, and therebymitigating the image resolution deterioration due to the electronscattering. The image-forming means comprises a plurality ofmicro-deflectors for forming the plurality of parallel virtual images,or a plurality of micro-deflector-and-compensator elements for formingthe plurality of parallel virtual images and compensating the off-axisaberrations of the plurality of probe spots.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A charged-particle beam apparatus, comprising: acharged particle source configured to provide a primary beam; an imageforming unit configured to form a plurality of images of the chargedparticle source using a plurality of beamlets of the primary beam; afirst projection system including a transfer lens and an objective lens,wherein the transfer lens is configured to focus the plurality ofbeamlets, and wherein the objective lens is configured to receive thefocused plurality of beamlets and form a plurality of probe spots on asample from the plurality of beamlets; a second projection systemconfigured to focus a plurality of secondary beams generated by theplurality of probe spots on the sample; a beam separator configured toseparate the plurality of beamlets and the plurality of secondary beams;and a detection device with a plurality of detection elements configuredto receive the plurality of secondary beams, wherein the secondprojection system includes an anti-rotation magnetic lens configured tominimize a rotation of the plurality of secondary beams directed to thedetection device.
 2. The charged-particle beam apparatus of claim 1,wherein the transfer lens is configured to focus the plurality ofbeamlets to land on the sample perpendicularly.
 3. The charged-particlebeam apparatus of claim 2, wherein the transfer lens is configured tofocus the plurality of beamlets to pass through a front focal point ofthe objective lens.
 4. The charged-particle beam apparatus of claim 1,wherein the transfer lens is configured to reduce variation of imagingmagnification by adjusting a focusing power of the transfer lens.
 5. Thecharged-particle beam apparatus of claim 4, wherein the transfer lens isconfigured to influence the trajectory of the corresponding beamlet byadjusting the focusing power.
 6. The charged-particle beam apparatus ofclaim 1, further comprising: a deflection scanning unit configured toscan the plurality of probe spots on the sample.
 7. The charged-particlebeam apparatus of claim 6, wherein the deflection scanning unit isconfigured to reduce off-axis aberrations of each off-axis beamlet byindividually optimizing a trajectory of a corresponding beamlet.
 8. Thecharged-particle beam apparatus of claim 7, wherein the deflectionscanning unit comprises a plurality of deflectors and each deflector ofthe plurality of deflectors is configured to reduce off-axis aberrationsof each off-axis beamlet by individually optimizing a trajectory of acorresponding beamlet.
 9. The charged-particle beam apparatus of claim1, further comprising: a beam-limiting element having a plurality ofbeam-limit openings for limiting the plurality of beamlets.
 10. Thecharged-particle beam apparatus of claim 1, further comprising: acondenser lens configured to focus the primary beam to vary electriccurrents of the plurality of probe spots on the sample.
 11. Thecharged-particle beam apparatus of claim 1, wherein the secondprojection system includes a zoom lens configured to minimize radialdisplacements of the plurality of secondary beams.
 12. Thecharged-particle beam apparatus of claim 11, wherein the zoom lensincludes a plurality of lenses.
 13. The charged-particle beam apparatusof claim 11, wherein the zoom lens and the anti-rotation magnetic lensare configured to focus the plurality of secondary beams onto thedetection elements.